Engine design and components


The engine is constructed from different parts. These parts are: bottom end, top end, front end, oil pan, valve cover, and front cover.

Bottom End (short block): The bottom end includes the cylinder block with all of its internal parts installed. The pistons, rods, crankshaft, and bearing would be in the block. The term short block is often used to mean the same thing as bottom end.

Long block: is a technician term that refers to the short block with just the heads installed. Parts like valve covers, front cover, flywheel, mounts, etc. are not included in the long block

Bare block: is a cylinder block with all parts removed. There would not be pistons, rods, a crankshaft, nor other parts in the block.


Bottom (Lower) End Construction


Cylinder block deck: is a flat machined surface for the cylinder head. Bolt holes are drilled and tapped in the deck for heat bolts. Coolant and oil passages allow fluids through the block, head gasket, and cylinder heads.

Cylinders (cylinder walls): are large holes machined in the cylinder block for the pistons. An integral cylinder is part of the block.

Cylinder block sleeves (liners): is a separate part pressed into the block. There are two basic types of cylinder sleeves: dry sleeves and wet sleeves.

Cylinder bores: There are several bores in the block, lifter bores, cam bores, main bore.

Main caps: they are blot to the bottom of the cylinder block and form one-half of the main bore. Large main cap bolts screw into holes in the block to secure the caps to the block

Main bearings: snap-fit into the cylinder block and main caps to provide an operating surface for the crankshaft main journals.

Crankshaft: It converts the reciprocating of the pistons into a rotating motion. The crankshaft fits into the main bore of the block The crankshaft has a crankshaft oil passages, counterweights, crankshaft flange with the pilot bearing in the center to support the manual transmission input shaft, and the crankshaft oil seals.

Crank main journals: are precision machined and polished surfaces that ride on the main bearings.

Crank rod journals: also terms crankpins are also machined and polished surfaces, but they are for the connecting rod bearings.

Flywheel: is a large steel disc mounted on the rear flange of the crankshaft. The flywheel has a large ring gear that allows engine starting.

Connecting rod: fastens the piston to the crankshaft.

Connecting rod cap: bolts to the bottom of the connecting rod body. It can be removed to for disassembly of the engine.

Connecting rod bearings: rid on the crankshaft rod journal.

Piston pin: allows the piston to swing on the connecting rod. The pin fits through the hole in the piston and the connecting rod small end.

Pistons: transfers the pressure f the combustion to the connecting rod and crankshaft. It must hold the piston rings and piston pin while operating in the cylinder.

Piston rings: Automotive pistons normally use three rings- two compression rings and one oil ring.

Balancer shafts: are used in some engines to reduce vibration. These counterweighted shafts are generally mounted in the left and right side of the engine block and are driven by a belt or chain.


Top (Upper) End Construction


* The cylinder head: bolts to the deck of the block and covers the top of the cylinders. A head gasket seals the block and head surfaces to prevent oil, coolant, and pressure leakage.

* Bare cylinder head: is a head casting, with all of its parts (valves, keepers, retainers, springs, seals, and rocker arms) removed. The cylinder head consists of combustion chambers, intakes ports, exhaust ports, oil passages, water jackets, intake deck, exhaust deck, and dowel holes.

* Valve guides: are small holes machined through the top of the head, do into the intake and exhaust ports. The two basic types of valve guides are the integral and pressed-in.

* Valve seats: are round, machined surfaces in the port openings to the combustion chambers. The valve seats can be part of the head or a separated pressed-in component.

* Valves: open and close to control flow in and out of the combustion chamber.
* Valve seals: prevent oil from entering the cylinder head ports through the valve guides.

* Valve spring assembly: is used to close the valve. It basically consists of a valve spring, retainer, and two keepers.

* Camshaft: opens the engine valves at the right time during each stroke.

* Camshaft gear: a camshaft sometimes has a drive gear for operating the distributor and oil pump.

* Camshaft eccentric: An eccentric (oval) may be machined on the camshaft for a mechanical (engine driven) fuel pump.

* Camshaft bearings: are usually one-piece inserts pressed into the block of cylinder head.

* Valve lifters: also called tappets, ride on the cam lobes and transfer motion to the rest of the valve train.

* Push rods: transfer motion between the lifters and the rocker arms. They are needed when the camshaft is located in the cylinder block

* Rocker arms: can be used to transfer motion from the push rods to the valves. They can be used in both OHC and OHV engines. In any engine, the rockers mount on top of the cylinder head by various methods; rocker shaft, rocker stud, or rocker pedestal. There are two types of rocker arms; adjustable rocker arms, and nonadjustable rocker arms. Adjustable rocker arms provide a means of changing valve train clearance. Nonadjustable rocker arms provide no means of changing valve clearance. They are only used with some hydraulic lifters.

* Solenoid-operated rockers: are used on variable displacement (size) engines. The solenoids can be turned on or off to deactivate or activate some of the engine valves.

* Variable Valve Timing: alter valve timing as engine speed changes. This is done to optimize engine power and efficiency at all operating speeds.


Front End Construction


Camshaft drive mechanism also called timing mechanism, must turn the camshaft and keep it in time with the engine crankshaft and pistons. Sometimes, it must also power other units (balancer shaft, oil pump, distributor, etc.) There are three basic types of camshaft drives: gear drive, chain drive, and belt drive.

Gear drive: Timing gears are two helical gears on the front of engine that operate the engine camshaft.

Timing chain and two sprockets: the timing chain transfers power from the crank sprockets to the cam sprockets. A crankshaft key is used to lock the crankshaft sprocket to the shaft. A camshaft key or dowel is used to secure the cam shaft sprocket on the cam, and assures that the sprocket does not spin on the camshaft and go out of time. A chain tensioner can be used to take up excess slack as the chain and sprockets wear. A chain guide may be needed to prevent chain slap. Auxiliary chain and sprockets may be used to drive the engine oil pump, balancer shafts, and other units on the engine. Oil slinger helps spray oil on the timing chain to prevent wear. Engine front cover, also called timing chain or timing gear cover, is a metal housing that bolts on the front of the engine. It encloses the timing chain or gears to keep oil from spraying out. The cover holds the crankshaft oil seal.

Timing belt: Teeth axe formed in the inside of the belt. They mesh with the teeth on the outside of the crank and cam sprockets. A belt sprocket normally has square teeth. Timing belt tensioner is a wheel that keeps the timing belt tight on its sprockets. Timing belt sensors detects excessive tensioner extension and timing belt wear and stretch. When the sensor detects belt stretch, an indicator of possible belt failure, it signals the ECU. The ECU can then illuminate a dash light to warn the driver of the problem. An auxiliary belt sprocket, also termed intermediate sprocket, can be used to operate the oil pump, water pump, distributor, etc. The timing belt simply extends around this extra sprocket. A timing belt cover is simply a sheet of metal or plastic shroud around the cam drive belt.

* Crankshaft pulleys: are needed to operate the alternator, power steering pump, air conditioning compressor, air injection pump, and other devices.

* Engine balance shafts: are geared to the crankshaft or camshaft. The balance shaft has bob weights that spin in the opposite direction of crankshaft rotation. This cancels out torsional vibrations created by the crankshaft, providing a smoother engine idle.

* Intake manifolds: is a metal casting or plastic part casting that bolts over and covers intake ports on the cylinder head.

Exhaust manifold bolts to the cylinder head, over the exhaust ports. Valve cover also called rocker cover or cam cover on OHC engines, is a thin housing over the top of the cylinder head. It simply keeps valve train oil spray from leaking out if the engine. The cover is sealed by a gasket or sealant.

Engine gaskets prevent pressure, oil, coolant, and air leakage between engine components. They are; cylinder head gasket, valve cover gasket, oil pan gasket, front cover gasket, thermostat housing gaskets, intake and exhaust manifold gaskets, etc.


Oil pan and sump


The oil pan, normally made of thin sheet of metal or aluminum, bolts to the bottom of the engine block. It holds an extra supply of oil for the lubrication system. The oil pan is fitted with screw-in drain plug for oil changes. The sump is the lowest area in the oil pan where oil collects.



Single- and multi- cylinder engines


Power/weight ratio:

The engine power varies as the square of the bore (that is, with the piston area) but the mass varies as the cube of the bore (that is, with the volume of material used). Increasing power by using a large cylinder therefore results in a low power/weight ratio, whereas increasing the number of cylinders maintains power and weight in the same proportions.


Firing interval and torque fluctuation:

Since all the cylinders must fire in two revolutions of the four-stroke crankshaft, the firing intervals is 7200 divided by the number of cylinders. The effective power stroke occupies about 1350. With a. single cylinder the mass of a large flywheel is required to absorb torque fluctuations and provide energy for the crankshaft. As the number of cylinders increases, torque is smoother and less weight flywheel is needed, aiding acceleration.



Large cylinders have long heat paths, such as from the piston center. Multi-cylinder units are necessary for large power to avoid lubrication and detonation problems due to overheating.


Balance and inertia loads:

The single-cylinder unit can only be imperfectly balanced and vibration will occur at certain engine speeds. Four-cylinder in-line units have small secondary out-of-balance forces, while horizontally opposed; six and eight-cylinder units can have entirely satisfactory balance. The reduced reciprocating mass of the multi-cylinder engine permits higher crankshaft speeds without inertia force problems.


Conventional car engine:

A part from the benefits of traditional experience in this type of unit, the four-stroke, four-cylinder, in-line, water-cooled engine has inherent advantages.

* The two-stroke unit has unacceptable fuel consumption.

* The economy of the compression-ignition (CI) is offset by the lower power and

acceleration, with increased cost, noise, weight and (to some) more objectionable fuel.

* Twin-cylinder engine have greater torque fluctuations, and

* Six cylinder units are an unnecessary expense under 2-2.5 L capacity.

* The V4 and the horizontally opposed four (HO4) are more expensive, and have a lot of components compared to the in-line layout, and H04 has complicated manifolds

and cooling arrangements.

* Air cooling in not suited to four-cylinder in-line units; it is noisier, requires power to drive the large cooling fan, and complicated interior heating